Nowadays information expands, so it is necessary to use a high density optical disc as a recording or memorizing device of music and image information of the computer memories, compact discs, video discs and the like.
As a one method of making high density, there is a process of enlarging an open number NA of the object lens of an optical pick up optically recording and reproducing information more than that of the conventional compact discs and video discs.
When the open or aperture number NA of the object lens is enlarged as described above, a comatic aberration of a spot focussed on the optical disc is enlarged in case the optical disc skewed. As a result, for example reproducing wave form strain is enlarge or increases, symbols are connected, and crosstalk is generated between memory tracks on the optical disc.
When a shape of the optical spot on an information recording surface of the optical disc reproducing apparently changes to an oval one along not only a radial direction but also a recording track direction, and further a long axis direction of the oval light spot coincides with an extending direction of the recording track, between-signals-interference is generated in a reproduced signal. Consequently, two pits of sequential pits are made near to each other in a high density recording resulting in an impossibility of correct or effective reproducing the recorded information.
On the contrary, when the long axis of the oval is coincided with a radial direction of the optical disc, crosstalk is generated between recording tracks. Consequently, when the recording is done at a high density narrowing the track pitch, it becomes impossible to correctly reproduce recorded information.
In addition, in the optical disc reproducing apparatus, a comatic aberration makes a point spread function and a line spread function of the light spot non-symmetrical. So, it becomes impossible to effectively correct any reproducing wave form strain by means of, for example, an ordinal fixed three-tap type equalizer.
That is, when the optical disc is used as a compact disc, for example, it has a transparent substrate of a thickness of about 1.2 mm. Light beam is shone and focussed on the information recording surface through the transparent substrate, and, for example, a reflected light from the recording surface reproduces information signals.
Consequently, when the information recording surface of the optical disc is skewed or inclined relative to light axis of the object lens, comatic aberration is generated proportioning to about three power of the aperture number NA and about one power of skew value.
The comatic aberration is expressed by Zaidel (phonetic)aberration coefficient equation as follows. ##EQU1##
Equation (1) is transferred to the following equation (2), ##EQU2## wherein, W31d is a ternary comatic aberration, Td is a thickness of the transparent substrate of the optical disc, Nd is a refractive ratio of the transparent substrate of the optical disc, and NAO is an aperture number of the object lens.
When NA is 0.6 and the aperture number is so large as 1.33 times of the compact disc, the comatic aberration generated is 2.37 times if the disc skew value is identical with that (aperture number NA=0.45) of the compact disc. In other words, when NA=0.45 of the compact disc, it is supposed that 0.6.degree. of the disc skew is permittable value of the disc skew value becomes so small considerably as of 0.25.degree..
By the way, it is practically difficult to make any disc skew completely remove or small because that warp or wave of the optical disc along its radial direction and tangential direction (circumferential direction), and inclination of the optical disc owing to the tilt of the disc mounting surface from the mounting standard surface.
When a wave face is strained owing to comatic aberration and the like, light strength distribution or spread function of the light spot focussed on the optical disc is made of non-symmetrical.
FIGS. 1(A), 1(B) to FIGS. 4(A), 4(B) respectively show a point spread function and line spread function of the light spot on the optical disc when the optical disc is tilted by 0.degree., 0.1.degree., 0.2.degree. and 0.3.degree.. Apparently from the figures, the larger the optical disc skews, the larger the value of non-symmetricity because strain of wave face rises the side robes of the light strength of the light spot in a manner of symmetrically.
Upwardly rising shown in FIG. 1(A) to FIG. 4(A) display linearly light strength of respective positions of the light spot on the disc flat surface within a shone surface of the spot. Curves shown in FIG. 1(B) to FIG. 4(B) display functionally light strength 0, -10, -20, -30, -40, -50 (axis of ordinate (dB)) at respective positions -1.5, -1.0, -0.5, 0, 0.5, 1.0, 1.5 (axis of abscissa (.mu.m)) of an imaginary line passing through a center of the light spot.
As described above, the disc skew along the tangential direction of the optical disc generates owing to rising of side robes of light strength distribution or spread functions of a light spot, and as a result interference between symbols increase considerably.
FIG. 5(A) and FIG. 6(A), respectively show examples of digital signals of reproducing signal wave forms (EFM reproduced wave forms) produced when skew along tangential direction (circumferential direction of the disc) is 0.degree. and 0.3.degree.. Comparing to each other of both examples, it is apparent that the case in which the disc skew is so large as 0.3.degree. has a larger wave form strain of EFM reproducing wave forms comparing to the case of 0.degree. resulting in a closing of eyepattern and a difficulty of picking-up correctly signals.
FIG. 5(A) and FIG. 6(A) show input signals of a fixed three-tap type equalizer of the optical disc apparatus, and FIG. 5(B) and FIG. 6(B) show output signals of the fixed three-tap type equalizer.
When analog signals are used and the disc skew along a tangential direction (circumferential direction) increases, C/N decreases and group delay phenomenon is generated. In particular, in case of video discs, there is a problem of appearing an uneven color on the display.
When a disc skew along a radial direction is generated, non-symmetrical spread function of the light spot or rising of side robes is happened resulting in large crosstalk because the wrong neighboring track is reproduced.
As shown in FIG. 7, concerning the crosstalk value of the disc skew along its radial direction, when the disc skew is .+-.0.3.degree., its crosstalk increases by 15 (dB) comparing to the case in which the disc skew is 0.degree..
That is, in anyone of the cases of digital signals and analog signals, when a disc skew along a radial direction increases, crosstalk increases and C/N decreases. In particular, concerning a video disc, and etc., there is a problem of appearing of horizontal synchronizing signals of the neighboring track (horizontal synchronizing signal of the neighboring track flows slowly horizontally).
In order to solve such problems of the conventional optical disc apparatus, a method controls an inclination of whole structure of an optical pick up in order to correct the skew of the disc. The conventional apparatus has a movable portion for controlling such inclination of the pick up and the movable portion is large in size and in weight, so it is difficult to have a quick and good responsibility.
In this connection, skew of about 0.1.degree. to 0.2.degree. generated during a rotation of the optical disc has a frequency so high as about several tenth to hundred several tenth (Hz), so that it is very difficult to control the heavy optical pick up and correct such skew of the optical disc.
In addition, in accordance with the conventional apparatus a skew servo motor of an equalizer shown in FIG. 8 is used to tilt the whole structure of an optical pick up relative to the optical disc along its radial direction in order to keep a light axis perpendicular to a surface of the optical disc. Thus, the skew servo motor functioned along only a radial skew direction electrically corrects such reproducing wave form strain.
The three-tap type equalizer EQ consisting of an equalizer of a transversal filter type of an equalizer fixed three-tap type delay circuit has delay circuits DL1 and DL2, fixed gain amplifying circuits AM1, AM2 and AM3, and adder circuit AD1. The delay circuits DL1 and DL2 are adapted to sequentially give unit delay value .tau. to input signals. Consequently, three delay signals h (t-.tau..sub.1), h (t) and h (t+.tau..sub.1), respectively having timings differed sequentially by an unit delay value .tau..sub.1 are multiplied by weighting coefficients a, 1 and a in the amplifying circuits AM1, AM2 and AM3, then they are added in an adder circuit AD1. The resultant added output H(t)=ah(t-.tau..sub.1)+h(t)+ah(t+.tau..sub.1) are outputted to the equalizer EQ.
In the conventional construction, considering first it on a frequency axis, when a modulation transfer function (MTF) expressing a frequency (defined by spacial frequency) characteristic of the optical pick up displays the feature shown in FIG. 9(A) at a skew angle 0.degree., the MTF is corrected by the three-tap type equalizer EQ having a right rising frequency characteristic shown in FIG. 9(C). Thus, as shown in FIG. 9(B), the frequency characteristic is improved by gain rises about a half of cut-off frequency.
When transmission function of the three-tap type equalizer EQ is H(t)=-ah(t-.tau..sub.1)+h(t)-ah(t+.tau..sub.1), it is expressed by using a spacial frequency f, an equation G(.omega.)=1-2 a cos (*.sub.1 .omega.) (.omega.=2.pi.f).
As shown in FIG. 10(A1) to FIG. 10(C), an improvement function of a frequency characteristic of this three-tap type equalizer EQ can be explained as an overlapping of curves of spread function of the light spot. That is, when line spread function of the light spot before correction is expressed as linear value of line spread function curve h(t), it is a symmetrical spread or distribution curve with a center of a time point t as shown in FIG. 10(A1). It can be expressed as logarithmic value thereof as shown in FIG. 10(B). Similarly, line spread function h(t) of the light spot after correction can be expressed by spread curves shown in FIG. 10(A2) and FIG. 10(B2).
However, light spots must be obtained at the output terminals of the three-tap type equalizer EQ, which spots having the line spread functions expressed by H(t)=-ah(t-.tau..sub.1)+h(t)-ah(t+.tau..sub.1). It can be determined as shown in FIG. 10(C) of a spread function H(t) (see FIG. 10(B1)) overlapped three line spread functions h(t), -ah(t-.tau..sub.1) and -ah(t+.tau..sub.1).
The overlapped line spread function curve h(t) is adapted to subtract two line spread function curves -ah (t-.tau..sub.1) and -ah(t+.tau..sub.1) having symmetrical side robes from the line spread function h(t). As a result, the resultant spread function curve has rising and falling portions more abrupt than that of line spread function curve h(t) by the amount subtracted. It is said that the frequency characteristic is improved to be steep.
However, when a disc skew is happened, as shown in FIGS. 11(A1) and 11(B1), 11(A2) and 11(B2) and 11(C) corresponding to FIGS. 10(A1) and 10(B1), 10(A2) and 10(B2) and 10(C), the line spread function curves -ah(t-.tau..sub.1), h(t) and -ah(t+.tau..sub.1) before correction have non-symmetrical shapes provided with a rising side robes.
Consequently, overlapping these spread function curves, the line spread function curves H(t) after correction resembles to substantially to the line spread function curve h(t) before correction.
As a result, it is recognized that it is impossible to obtain correction effect of the disc skew happened by means of the three-tap type equalizer EQ.
As described above, the conventional optical disc apparatus has the following problems.
First, the structure for mechanically tilting the optical pick up is complicated and decreasing the manufacturing cost of the structure is difficult.
Second, the structure for mechanically tilting the optical pick up is heavy and it is impossible to expect a high speed or quick response.
Third, it is impossible to do skew servo along the tangential direction or circumferential direction.
Fourth, the three-tap type equalizer can not sufficiently correct reproducing wave form strain due to disc skew correct reproducing wave form strain due to disc skew.